The invention relates to the field of pumping fluids and, more particularly, to a system for pumping oil and gas from a subterranean well.
Conventional pumping systems utilize reciprocating subsurface pumps in order to produce fluids, namely oil and entrained and/or free gas, from a subterranean well to the surface. FIG. 1 illustrates a typical conventional system wherein fluids are passed through a downhole separator, separated gas is produced through the annular space defined between a production tube and a well casing of the subterranean well, and separated oil is produced through the production tube.
Reciprocating subsurface pumps are actuated by a rod string which passes through the production tube for connection with the subsurface pump. In situations where the oil produced is a viscous crude oil, the rod string is subjected to excessive friction and fatigue due to the viscous crude oil, which may cause rod failure and other serious problems such as damage to the pump, the production tube, the gear box and/or the surface stuffing box. Further, the increased friction on the rod string necessitates extra force to drive the rod string which in turn contributes further to the excess wear on the rod string. Additionally, viscous oil has the tendency to reduce the falling speed of the rod string due to flotation or buoyancy effects. This, of course, results in a decrease in pumping speed and a decrease in production and efficiency.
Numerous proposals have been made for dealing with the problem of reducing the friction and fatigue to which the rod string is subjected. FIG. 2 illustrates one proposed solution wherein low viscosity fluid is pumped down through the annular space to mix with heavy crude oil being produced through the production tube. The mixture has a reduced viscosity which results in less friction on the rod string. Nevertheless, this system requires the additional power required to pump the low viscosity fluid down the annular space, and also results in the production of an oil/low viscosity fluid mixture which must be separated at the surface. Additionally, typical subsurface pumps suffer from a decrease in pumping efficiency when operated on a mixture of liquid and gas. In the system of FIG. 2, the pump must act on oil, entrained gas, and the low viscosity fluid which may be additional gas, resulting in a loss of efficiency of the pump.
FIG. 3 illustrates another proposed solution wherein a low viscosity fluid is pumped through the production tube to mix with pumped oil and pass through a slotted pipe section to be produced through the annular space. As with the proposal of FIG. 2, however, additional power is required to pump the low viscosity fluid. Also, produced oil/fluid mixtures must be separated, and the subsurface pump must still act on oil and any entrained and/or free gas carried with the oil, thus reducing the efficiency of the system.
FIG. 4 illustrates another proposed solution wherein a shoe is disposed within the production tube and is sealed with a stuffing box so that oil and free and/or entrained gas produced through the pump are passed to the annular space for production. In the meantime, a low viscosity fluid is circulated through the production tube. This provides a reduction of the friction to which the rod string is subjected. However, the pump must still act on an oil/gas mixture resulting in a reduction in pumping efficiency. Further, fluids produced must still be separated at the surface.
It is desirable, therefore, to provide a system for pumping fluids wherein the rod string is not subjected to excessive friction, and wherein the subsurface pump does not suffer a loss in efficiency due to the pumping of gas along with oil, and further wherein produced oil, gas, and other fluids do not need to be separated at the surface.
It is, therefore, a principal object of the present invention to provide a pumping system wherein oil is produced through the annular space and gas is produced through the production tube so as to reduce or eliminate the effects of friction on the rod string.
It is a further object of the present invention to provide such a system which does not require the pumping of additional fluids into the well.
It is a still further object of the invention to provide a pumping system wherein oil and gas are separated downhole and produced separately so as to avoid the necessity of surface separation.
Other objects and advantages will appear hereinbelow.